Ion implantation systems are used to dope semiconductors with impurities in integrated circuit manufacturing. In such systems, an ion source ionizes a desired dopant element, which is then extracted from the ion source in the form of an ion beam. The ion beam is typically mass analyzed to select ions of a desired charge-to-mass ratio and then directed at the surface of a semiconductor workpiece in order to implant the workpiece with the dopant element. The ions of the beam penetrate the surface of the workpiece to form a region of desired conductivity, such as in the fabrication of transistor devices in the workpiece. A typical ion implanter includes the ion source for generating the ion beam, a beamline assembly including the mass analysis apparatus for mass resolving the ion beam using magnetic fields, and a target chamber containing the semiconductor wafer or workpiece to be implanted by the ion beam. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station.
The mass of an ion relative to its charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely, by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor workpiece or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis mentioned supra. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway.
In order to achieve a desired implantation for a given application, the dose and energy of the implanted ions may be varied. The ion dose controls the concentration of implanted ions for a given semiconductor material. Typically, high current implanters are used for high dose implants, while medium current implanters are used for lower dose applications. The ion energy is used to control junction depth in semiconductor devices, where the energy levels of the beam ions determine the degree to which ions are implanted or the depth of the implanted ions within the semiconductor or other substrate material. The continuing trend toward smaller semiconductor devices requires a mechanism that serves to deliver high beam currents at low energies. The high beam currents provide the necessary dose levels, while the low energy currents permit shallow implants.
In most prior art systems, the ion implantation employed is a pencil-type ion beam, wherein a relatively narrow beam is produced by the ion source and subjected to mass analysis, subsequent beam conditioning, and scanning before reaching the workpiece. In this case, the reduced energies of the ions cause some difficulties in maintaining convergence of the ion beam due to the mutual repulsion of ions bearing a like charge. High current ion beams typically include a high concentration of similarly charged ions that tend to diverge due to mutual repulsion. One solution to the above problem is to employ a ribbon-type ion beam instead of a pencil-type beam. One advantage of the ribbon-type beam is that the cross-sectional area of the beam is substantially larger than the pencil-type beam. For example, a typical pencil beam has a diameter of about 1-5 cm, wherein a ribbon-type beam may have a height of about 1-5 cm and a width of about 40 cm. With the substantially larger beam area, a given beam current has substantially less current density, and the beam has a lower perveance. Use of a ribbon-type beam, however, has a number of unique challenges associated therewith.
Typical ribbon ion beam systems often have difficulty in preventing tight beam spots within the system which forces the space charge expansion of the ion beam to remain low. For example, referring to FIGS. 1 and 2 is a prior art approach from U.S. Pat. No. 5,126,575 for Method and Apparatus for Broad Beam Ion Implantation issued to Nicholas R. White. It is apparent in FIGS. 1 and 2 that the ion beam 1 has a tight beam spot located within the analyzing magnet 3 at a vertical focal point 12. (See also e.g., Col. 4, lines 4-18). The “tightness” of the beam 1 at the focal point 12 allows the substantial loss of beam quality in a low energy beam due to space charge expansion which will result in poor beam transmission. In addition, referring to FIG. 3 is a prior art approach illustrated in U.S. Pat. No. 6,635,880 for High Transmission, Low Energy Beamline Architecture for Ion Implanter issued to Anthony Renau. In that prior art approach the ion beam 12 has a tight beam spot at the ion source 10 exit, for example. That prior art approach is known to have limitations in providing high current, low energy ion beams.
Therefore, in ribbon ion beam implantation systems, there remains a need for a ribbon ion beam implantation system that provides high beam currents at low energies.